FIG. 8 is a schematic view showing the general structure of a plasma etching apparatus. The plasma etching apparatus has an airtight and cylindrical process chamber 50. A worktable (used also as a lower electrode) 51 for supporting a wafer W handled as a target substrate is disposed (e.g. movable up and down) on the lower side in the process chamber 50. A showerhead (used also as an upper electrode) 52 for supplying a process gas is disposed on the upper side in the process chamber 50 and faces the lower electrode 51 in parallel therewith. The two electrodes 51 and 52 are respectively supplied with RF (radio frequency) powers different in frequency from first and second RF power supplies 53 and 54 through matching circuits 53A and 54A. With the RF powers applied to the electrodes 51 and 52, an RF electric field is formed in the process chamber 50. The RF electric field helps the process gas turn into plasma, which then etches a film, such as an insulating film, on the surface of the wafer W.
A focus ring 55 is disposed on the peripheral portion of the lower electrode 51 and surrounds the wafer W on the lower electrode 51. On the other hand, a shield ring 56 is disposed on the peripheral portion of the upper electrode 52. The focus ring 55 and shield ring 56 work on plasma generated between the upper and lower electrodes 51 and 52 to focus toward the wafer W.
In the conventional plasma etching apparatus, problems have been found in that abnormal electric discharge tends to occur on the peripheral portion of a wafer W, and lowers the etching planar uniformity and/or selectivity.